Method of forming a pixel sensor cell for collecting electrons and holes

ABSTRACT

The present invention is a pixel sensor cell and method of making the same. The pixel sensor cell approximately doubles the available signal for a given quanta of light. The device of the present invention utilizes the holes produced by impinging photons in a pixel sensor cell circuit. A pixel sensor cell having reduced complexity includes an n-type collection well region formed beneath a surface of a substrate for collecting electrons generated by electromagnetic radiation impinging on the pixel sensor cell and a p-type collection well region formed beneath the surface of the substrate for collecting holes generated by the impinging photons. A circuit structure having a first input is coupled to the n-type collection well region and a second input is coupled to the p-type collection well region, wherein an output signal of the pixel sensor cell is the magnitude of the difference of a signal of the first input and a signal of the second input.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pixel sensors and in particular a pixelsensor which approximately doubles the available signal for a givenquanta of light.

2. Description of Related Art

CMOS image sensors are beginning to replace conventional CCD sensors forapplications requiring image pick-up such as digital cameras, cellularphones, PDA (personal digital assistant), personal computers, and thelike. Advantageously, CMOS image sensors are fabricated by applyingpresent CMOS fabricating process for semiconductor devices such asphotodiodes or the like, at low costs. Furthermore, CMOS image sensorscan be operated by a single power supply so that the power consumptioncan be restrained lower than that of CCD sensors, and further, CMOSlogic circuits and like logic processing devices are easily integratedin the sensor chip and therefore the CMOS image sensors can beminiaturized.

Current CMOS image sensors comprise an array of pixel sensor cells,which are used to collect light energy and convert it into readableelectrical signals. Each pixel sensor cell comprises a photosensitiveelement, such as a photodiode, photo gate, or photoconductor overlying adoped region of a substrate for accumulating photo-generated charge inan underlying portion thereof. A read-out circuit is connected to eachpixel cell and often includes a diffusion region for receiving chargefrom the photosensitive element, when read-out. Typically, this isaccomplished by a transistor device having a gate electrically connectedto the floating diffusion region. The imager may also include atransistor, having a transfer gate, for transferring charge from thephotosensitive element to the floating diffusion region, and atransistor for resetting the floating diffusion region to apredetermined charge level prior to charge transfer.

As shown in FIG. 1, a typical CMOS pixel sensor cell 10 includes apinned photodiode 20 having a pinning layer 18 doped p-type and anunderlying collection well 17 lightly doped n-type. P-type pinning layer18 is electrically coupled to p-type substrate 15 by a doped p-typeregion 29. Typically, pinned photodiode 20 is formed on top of a p-typesilicon substrate 15, or a p-type epitaxial silicon layer or p-wellsurface layer, having a lower p-type concentration than pinning layer18. N region 17 and p region 18 of photodiode 20 are typically spacedbetween an isolation region 19 (i.e. shallow trench isolation (STI)) anda charge transfer transistor gate 25 which is surrounded by thin spacerstructures 23 a,b. The photodiode 20 thus has two p-type regions 18 and15 having a same potential so that the n region 17 is fully depleted ata pinning voltage (Vp). The pinned photodiode 20 is termed “pinned”because the potential in the photodiode 20 is pinned to a constantvalue, Vp, when the photodiode 20 is fully depleted.

In operation, electromagnetic radiation 35 (i.e. visible light)impinging the pixel is focused down onto the photodiode 20 creatingelectron-hole pairs 40. Electrons collect at the n type region 17. Whenthe transfer gate structure 25 is operated, i.e., turned on, thephoto-generated charge 24 is transferred from the charge accumulatinglightly doped n-type region 17 via a transfer device surface channel 16to a floating diffusion region 30 which is doped n+type. Charge ondiffusion region 30 is eventually transferred to circuit structure 45(i.e. source-follower gate) for amplification. In the conventional pixelsensor cell 10, holes equal in number to the electrons are generated bythe impinging electromagnetic radiation 35 and are collected in thesubstrate 15. Since the substrate 15 is usually grounded, the holes exitthe pixel sensor cell 10 through the ground line 50. Thus, only abouthalf of the generated carriers (i.e. electrons) are collected tocontribute to the output signal of the conventional pixel sensor cell.

In commonly assigned U.S. Pat. No. 6,194,702 filed on Aug. 25, 1997 toHook et al. (hereinafter referred to as “Hook”), two complementarycircuits (i.e. PFET and NFET circuits) are created for use with thephotodiode with each circuit: capturing electrons and holes,respectively. Portions of the electron and hole collection wells areused as source/drain regions for the respective NFET and PFET devices.The formation of active devices such as the NFET and PFET transistors inthe photodiode reduces the area available to collect impingingelectromagnetic radiation. Also, the process complexity is increasedsince additional films must be deposited and etched to form the NFET andPFET. Exposure of the upper substrate surface of the photodiode to theadditional fabrication steps increases defects at the substrate surfaceof the photodiode which can have adverse effects on the pixel sensorcell of Hook such as an increase in dark current.

Bearing in mind the problems and deficiencies of the prior art, what isrequired is a pixel sensor cell having reduced complexity for collectingboth electrons and holes.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a pixel sensor cellhaving reduced complexity for collecting both electrons and holes.

It is another aspect of the present invention to provide a pixel sensorcell with improved signal-to-noise ratio.

It is another aspect of the present invention to provide a pixel sensorcell with a reduced size collection area.

A further aspect of the invention is to provide a pixel sensor cellwhich approximately doubles the output current relative to conventionalpixel sensor cells.

It is yet another aspect of the present invention to provide a pixelsensor cell which virtually eliminates substrate current.

Still other aspects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The above and other objects and advantages, which will be apparent toone of skill in the art, are achieved in the present invention which isdirected to, in a first aspect, a pixel sensor cell and method thereofcomprising an n-type collection well region formed beneath a surface ofa substrate for collecting electrons generated by electromagneticradiation impinging on the pixel sensor cell and a p-type collectionwell region formed beneath the surface of the substrate for collectingholes generated by the impinging electromagnetic radiation. A circuitstructure having a first input is coupled to the n-type collection wellregion and a second input is coupled to the p-type collection wellregion, wherein an output signal of the pixel sensor cell is themagnitude of the difference of a signal of the first input and a signalcf the second input.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a prior art pixel sensor cell.

FIG. 2 is a cross-sectional view of an embodiment of the pixel sensorcell according to the present invention.

FIGS. 3A-D illustrate exemplary steps for producing the pixel sensorcell of the present invention.

FIGS. 4-6 are cross-sectional views of various other embodiments of thepixel sensor cell according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing embodiments of the present invention, reference will bemade herein to FIGS. 1-6 of the drawings in which like numerals refer tolike features of the invention. Features of the invention are notnecessarily shown to scale in the drawings.

Embodiments of the invention are described herein below in terms of a“pixel sensor cell”. It is noted that the term “pixel sensor cell” isused to generally refer to any type of sensor cell which is capable ofconverting incident electromagnetic radiation into an electrical signal.An example of a pixel sensor cell according to the invention includes apixel sensor cell that is capable of detecting optical wavelengths ofelectromagnetic radiation and is commonly referred to as an “imagesensor”. An image sensor fabricated using CMOS technology is commonlyreferred to as a “CMOS image sensor”.

FIG. 2 illustrates a schematic cross-section of an embodiment of thepixel sensor cell 100 of the present invention. As shown in FIG. 2, aphotosensitive device or photodiode 200 is formed in a substrate 15 of afirst conductivity type, e.g. p type, between isolation regions 19 (e.g.STI). The photodiode 200 comprises a surface pinning layer 180 dopedwith material of a second conductivity type, e.g., n type materialdopant, a first charge collection well region 175 doped with material ofa first conductivity, e.g., p type material dopant, formed directlyunderneath the pinning layer 180, and a second charge collection wellregion 170 doped with material of a second conductivity, e.g. n typematerial dopant, formed directly underneath the first charge collectionwell region 175. The pinning layer 180 is schematically shownelectrically coupled to voltage source V1, e.g. Vdd for n type pinninglayer 180 (ground for a p type pinning layer 180). The pinning layer 180can be contacted by, for example, an interconnect (i.e. wire) whichprovides an electrical path to the voltage source V1. Optionally, thepinning layer 180 can be omitted. A first contact region 171 doped withmaterial of a second conductivity type, e.g. n type dopant material, andan interconnect such as a copper (Cu) or aluminum wire (shownschematically) in contact with the first contact region 171 electricallyconnect the n collection well region 170 to an input 190A of circuitstructure 195. A second contact region 176 doped with material of aFirst conductivity type, e.g. p type dopant material, and aninterconnect such as a copper (Cu) or aluminum wire (shownschematically) in contact with the second contact region 176electrically connect the p collection well region 175 to an input 190Bof circuit structure 195.

The photodiode 200 collects impinging electromagnetic radiation 35 (i.e.photons) which creates electron-hole pairs 40. The n collection wellregion 170 and the p collection well region 175 are initialized to apositive and a negative potential respectively, and are typicallydepleted. The electrons are collected in the n collection well region170 and flow to input 190A through the n contact region 171. The holesare collected in the p collection well region 175 and flow to input 190Bthrough the p contact region 176. A function of circuit structure 195 isto amplify a resultant signal created by the electrons and holes. Assuch, circuit structure 195 comprises a differential amplifier circuit197 having inputs coupled to inputs 190A, 190B. Additionally, circuitstructure 195 comprises a reset signal R coupled to switches T1 and T2(i.e. field effect transistors) for resetting the pixel sensor cell 100.For the polarities of the collection well regions 170, 175 describedhereinabove, T1 is an NFET and V3=Vdd, and T2 is a PFET and V2 ground(i.e. 0V). Since the signal created by the flow of electrons is acomplement of the signal created by the flow of holes, an output signalVout of the pixel sensor cell 100 will be approximately double that ofconventional pixel sensor cells using similarly sized photodiode regions(i.e. sensor 10 shown in FIG. 1). The pixel sensor cell 100 utilizes theholes that are generated by impinging electromagnetic radiation 35rather than allowing the holes to collect in the substrate 15 and exitthrough a ground connection as is done in conventional pixel sensorcells.

The additional signal from the holes will provide several advantages: 1)the additional signal may be used to improve the signal-to-noise ratiofor a given collection area of the photodiode 200; and 2) the additionalsignal may be used to reduce the collection area of the photodiode 200by approximately 50% in order to reduce the size of the pixel sensorcell 100. Since both the electrons and the holes can be collected andutilized in the circuit structure 195, the size of the collection areaof the pixel sensor cell 100 can be reduced by approximately 50% of thesize of the collection area of the conventional pixel sensor cell 10while producing output of similar magnitude to the conventional pixelsensor cell 10.

A method to fabricate a pixel sensor cell according to an embodiment ofthe invention will be described with reference to FIGS. 3A-D. As shownin FIG. 3A, there is provided a substrate 15 which may be a bulksemiconductor including, for example, Si, SiGe, SiC, SiGeC, GaAs, InP,InAs and other semiconductors, or layered semiconductors such assilicon-on-insulators (SOI), SiC-on-insulator (SiCOI) or silicongermanium-on-insulators (SGOI). For purposes of description of thisembodiment of the invention, substrate 15 is a Si-containingsemiconductor substrate of a first conductivity type, e.g., lightlydoped with p-type dopant material such as boron or indium (beryllium ormagnesium for a III-V semiconductor), to a standard concentrationranging between about 1×10¹⁴ atoms per cm³ to about 1×10¹⁶ atoms percm³. The light doping is advantageous for large depletion regions andgood light collection. Isolation regions 19 are formed in thep-substrate 15 by conventional methods known in the art.

Still referring to FIG. 3A, a masking layer such as photoresist isformed atop substrate 15 and is patterned to form ion implantation mask210 according to techniques known in the art to provide an opening 215between isolation regions 19 where the charge accumulation region of thephotodiode 200 is to be formed. The opening 215 permits the implantationof ions 220A of an n type dopant material to form the pinning layer 180.An example of the n type dopant material is phosphorous, arsenic orantimony. Phosphorous can be ion implanted at a concentration sufficientto form the n type pinning layer 180. For example, phosphorous can beion implanted at a substantially vertical angle in relation to thesurface of the substrate 15 at conditions of a dose from about 5×10¹²atoms per cm² to about 5×10¹³ atoms per cm² and an ion implant energyfrom about 5 keV to about 30 keV resulting in a phosphorousconcentration in a silicon substrate of about 1×10¹⁸ atoms per cm³ toabout 1×10¹⁹ atoms per cm³.

Referring to FIG. 3B, using the same ion implantation mask 210 (or,optionally, a different ion implantation mask), the opening 215 permitsthe implantation of ions 220B of a p type dopant material to form thecharge collection well region 175 beneath the n type pinning layer 180.The p type dopant material can be ion implanted at higher energy levelsto form the p type collection well region 175 of the photodiode 200 asshown in the FIGs. An example of the p-type dopant material is boron.Boron can be ion implanted at a (concentration sufficient to form the ptype collection well region 175. For example, boron can be ion implantedat a substantially vertical angle in relation to the surface of thesubstrate 15 at conditions of a dose from about 2×10¹² atoms per cm² toabout 2×10¹³ atoms per cm² and an ion implant energy from about 50 keVto about 150 keV resulting in a boron concentration in a siliconsubstrate of about 5×10¹⁶ atoms per cm³ to about 5×10¹⁷ atoms per cm³.

Still referring to FIG. 3B, using the same ion implantation mask 210(or, optionally, a different ion implantation mask), the opening 215permits the implantation of ions 220C of an n type dopant material toform the charge collection well region 170 beneath the p type collectionwell region 175. The opening 215 permits the implantation of ions 220Cof an n type dopant material to form the charge collection well region170. An example of the n type dopant material is phosphorous, arsenic orantimony. Phosphorous can be ion implanted at a concentration sufficientto form the n type collection well region 170. For example, phosphorouscan be ion implanted at a substantially vertical angle in relation tothe surface of the substrate 15 at conditions of a dose from about2×10¹² atoms per cm² to about 2×10¹³ atoms per cm² and an ion implantenergy from about 100 keV to about 1000 keV resulting in a phosphorousconcentration in a silicon substrate of about 5×10¹⁶ atoms per cm³ toabout 5×10¹⁷ atoms per cm³.

Ion implantation mask 210 is removed and ion implantation mask 230 isformed on substrate 15 to provide an opening 235 as shown in FIG. 3C.The opening 235 permits the implantation of ions 240 of an n type dopantmaterial to form the contact region 171 extending through the pinninglayer 180/p collection well region 175 and contacting the n collectionwell region 170. An example of the n type dopant material isphosphorous, arsenic or antimony. Phosphorous can be ion implanted at aconcentration sufficient to form the n type contact region 171. Forexample, phosphorous can be ion implanted at a substantially verticalangle in relation to the surface of the substrate 15 at conditions of adose from about 2×10¹⁴ atoms per cm² to about 3×10¹⁵ atoms per cm² andan ion implant energy from about 100 keV to about 300 keV resulting in aphosphorous concentration in a silicon substrate of about 5×10¹⁸ atomsper cm³ to about 5×10¹⁹ atoms per cm³.

Ion implantation mask 230 is removed and ion implantation mask 250 isformed on substrate 15 to provide an opening 255 as shown in FIG. 3D.The opening 255 permits the implantation of ions 260 of a p type dopantmaterial to form the contact region 176 extending through the pinninglayer 180 and contacting the p collection well region 175. An example ofthe p-type dopant material is boron. Boron can be ion implanted at aconcentration sufficient to form the p type contact region 176. Forexample, boron can be ion implanted at a substantially vertical angle inrelation to the surface of the substrate 15 at conditions of a dose fromabout 2×10¹⁴ atoms per cm² to about 3×10¹⁵ atoms per cm² and an ionimplant energy from about 10 keV to about 50 keV resulting in a boronconcentration in a silicon substrate of about 5×10¹⁸ atoms per cm³ toabout 5×10¹⁹ atoms per cm³.

It is noted that the steps described herein above with reference toFIGS. 3A-3D can be performed in a different order. That is, for example,the pinning layer 180 can be formed after the collection well regions170, 175 are formed. Likewise, contact region 176 can be formed beforecontact region 171.

Fabrication of devices, contacts and interconnects associated withcircuit structure 195 which are shown schematically in FIG. 2 can beachieved using conventional methods and, as such, fabrication detailsare omitted for the sake of clarity in order to maintain the focus ofthe description on the present invention. An advantage of the presentinvention is that the circuit structure 195 can be located away from thephotodiode 200 so that the collection area available for receivingimpinging electromagnetic radiation 35 can be increased. Contact regions171, 176 allow the electron and hole current flows to be routed to thecircuit structure 195 without the formation of active devices such astransistors in the vicinity of the collection area of the photodiode 200as described in commonly assigned U.S. Pat. No. 6,194,702 to Hook et al.Hook requires two transistors to be formed in the collection area inorder to provide the electron and hole current flows. In addition to theadded complexity to form the transistors, Hook exposes the uppersubstrate surface of the photodiode to fabrication steps such as filmdeposition/etching which increases defects at the substrate surface ofthe photodiode which can have adverse effects on the pixel sensor cellsuch as an increase in dark current.

FIG. 4 illustrates a schematic cross-section of another embodiment ofthe pixel sensor cell 100 of the present invention. The pixel sensorcell 100 shown in FIG. 2 is formed on a layered substrate 150 such assilicon-on-insulator (SOI), silicon carbide-on-insulator (SiCOI) orsilicon germanium-on-insulator (SGOI). For purposes of description ofthis embodiment of the invention, substrate 150 is a Si-containingsemiconductor substrate of a first conductivity type having a buriedinsulator layer 155 such as, for example, silicon oxide formed by aSIMOX (Separation by Implantation of Oxygen) process or otherconventional method. The pixel sensor cell 100 shown in FIG. 4 can befabricated using the processing steps as described herein above withreference to FIGS. 3A-D to form the photodiode 200 in a layer ofsubstrate 150 over the buried oxide layer 155. An advantage of the pixelsensor cell 100 shown in FIG. 4 is that each cell is electricallyisolated from adjacent pixel sensor cells so carriers that are generatedin the substrate from one pixel sensor cell cannot be collected byanother pixel sensor cell. This, “cross-talk” between pixel sensor cellsformed on an SOI substrate is reduced compared to pixel sensor cellsformed on a bulk substrate.

FIG. 5 illustrates a schematic cross-section of another embodiment of apixel sensor cell 100A of the present invention. As shown in FIG. 5, aphotodiode 200A is formed in a layer of substrate 150 over the buriedoxide layer 155. The photodiode 200A comprises a pinning layer 180 dopedwith material, e.g., p type material dopant, which substantiallysurrounds an n type charge collection well region 170 and a p typecharge collection well region 175. The pinning layer 180 isschematically shown electrically coupled to voltage source V1, e.g.ground for p type pinning layer 180 (Vdd for an n type pinning layer180). The pinning layer 180 can be contacted by, for example, aninterconnect (i.e. wire) which provides an electrical path to thevoltage source V1. An n type contact region 171 and an interconnect(shown schematically) electrically connect the n collection well region170 to an input 190A of circuit structure 195. A p type contact region176 and an interconnect (shown schematically) electrically connect the pcollection well region 175 to an input 190B of circuit structure 195.The pinning layer 180 as shown in FIG. 5 may result in a furtherreduction in dark current for the pixel sensor cell 100A compared to thepixel sensor cell 100 shown in FIG. 2 and conventional pixel sensorcells since the pinning layer 180 provides additional passivation alongsidewalls of the isolation regions 19. The pixel sensor cell 100A can befabricated using conventional methods (not shown) such as, for example,photolithography and ion implantation to form the various dopant regionsof the photodiode 200A.

FIG. 6 illustrates a schematic cross-section of yet another embodimentof a pixel sensor cell 100B of the present invention. As shown in FIG.6, a photodiode 200B is formed in a layer of the substrate 150 over theburied oxide layer 155. The photodiode 200B comprises a collection wellregion 175 doped with material, e.g., p type material dopant, whichsubstantially surrounds an n type charge collection well region 170. Ann type contact region 171 and an interconnect (shown schematically)electrically connect the n collection well region 170 to an input 190Aof circuit structure 195. A p type contact region 176 and aninterconnect (shown schematically) electrically connect the p typecollection well region 175 to an input 190B of circuit structure 195. Aswas described with reference to FIG. 2, the n type charge collectionwell region 170 collects electrons and the p collection well region 175collects holes. The pixel sensor cell 100B can be fabricated usingconventional methods (not shown) such as, for example, photolithographyand ion implantation to form the various dopant regions of thephotodiode 200B.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

1-11. (canceled)
 12. A method of forming a pixel sensor cell comprising:providing a substrate; forming an n-type collection well region in saidsubstrate for collecting electrons generated by electromagneticradiation impinging on said pixel sensor cell; forming an n-typediffusion region in direct physical contact with said n-type collectionwell region; forming a p-type collection well region in said substratefor collecting holes generated by said impinging electromagneticradiation; forming a p-type diffusion region in direct physical contactwith said p-type collection well region; and forming a circuit structurehaving a first input coupled to said n-type collection well region and asecond input coupled to said p-type collection well region, wherein anoutput signal of said pixel sensor cell being the magnitude of thedifference of a signal of said first input and a signal of said secondinput.
 13. The method of claim 12, wherein said steps of forming saidn-type and p-type collection well regions comprise: forming a maskstructure on said substrate having an opening; ion implanting an n-typedopant in said substrate exposed by said opening to form said n-typecollection well region; and ion implanting a p-type dopant in saidsubstrate exposed by said opening to form said p-type collection wellregion.
 14. The method of claim 13, wherein said step of ion implantingsaid n-type dopant comprises ion implanting phosphorous ions at asubstantially vertical angle in relation to the surface of saidsubstrate at a dose from about 2×10¹² atoms per cm² to about 2×10¹³atoms per cm² and an ion implant energy from about 100 keV to about 1000keV.
 15. The method of claim 13, wherein said step of ion implantingsaid p-type dopant comprises ion implanting boron ions at asubstantially vertical angle in relation to the surface of saidsubstrate at a dose from about 2×10¹² atoms per cm² to about 2×10¹³atoms per cm² and an ion implant energy from about 50 keV to about 150keV.
 16. (canceled)
 17. The method of claim 12, wherein said steps offorming said n-type and p-type diffusion regions comprise: forming amask structure having an opening on said substrate; ion implanting ann-type dopant in said substrate exposed by said opening to form saidn-type diffusion region; and ion implanting a p-type dopant in saidsubstrate exposed by said opening to form said p-type diffusion region.18. The method of claim 17, wherein said step of ion implanting saidn-type dopant comprises ion implanting phosphorous ions at asubstantially vertical angle in relation to the surface of saidsubstrate at a dose from about 2×10¹² atoms per cm² to about 2×10¹³atoms per cm² and an ion implant energy from about 100 keV to about 1000keV.
 19. The method of claim 17, wherein said step of ion implantingsaid p-type dopant comprises ion implanting boron ions at asubstantially vertical angle in relation to the surface of saidsubstrate at a dose from about 2×10¹² atoms per cm² to about 2×10¹³atoms per cm² and an ion implant energy from about 50 keV to about 150keV.
 20. The method of claim 12 further comprising the steps of: forminga first interconnect for coupling said n-type diffusion region to saidfirst input of said circuit structure; and forming a second interconnectfor coupling said p-type diffusion region to said second input of saidcircuit structure.
 21. The method of claim 12, wherein said step offorming said circuit structure comprises forming a differentialamplifier circuit having a negative input terminal coupled to said firstinput of said circuit structure and a positive input terminal coupled tosaid second input of said circuit structure.
 22. The method of claim 12further comprising forming a pinning layer in an upper surface of saidsubstrate.
 23. The method of claim 12, wherein said substrate comprisesa buried dielectric layer, said n-type and p-type collection wellregions formed in a portion of said substrate over said burieddielectric layer.
 24. The method of claim 12 further comprising a stepof forming a pinning layer substantially surrounding said n-type andp-type collection well regions.
 25. The method of claim 12, wherein oneof said n-type and p-type collection well regions is formedsubstantially surrounding the other n-type or p-type collection wellregions. 26-28. (canceled)